The city that never sleeps is turning out the lights on dozens of skyscrapers in the hope of protecting birds distracted from migration paths. Every night in autumn, hundreds collide with Manhattan's high-rise towers.

Skyscrapers are being urged to help save birds, and energy

Woodpeckers prefer New York City at night. And so do sparrows, warblers, thrushes, and other near-tropical migratory birds, soaring high above Manhattan after sunset on the way south.

But this autumn the Big Apple is trying to be more accommodating to the fearless migrants.

For the first time about 100 buildings - including the Empire State, Chrysler, City Group Center and others - are taking part in the Lights Out initiative.

Owners of tall buildings are being urged to dim their lights to save the lives of night-migrating birds, while reducing energy costs.

Dr Daniel Klem Jr, an ornithologist at Muhlenberg College in Pennsylvania, says up to a billion birds die in collisions with glass windows in the US each year, both during the day and at night.

Most of these collisions occur at the ground level during daytime, when birds mistake reflections in the windows for the trees or open space and fly into them.

But during migration season, illumination in the high-rise buildings also poses a threat, especially on nights without the strong winds that help guide birds.

Empire State research

Some birds are attracted to the lights and on cloudy nights with light winds can start circling the buildings, and may die colliding with it or fall exhausted to the street.

Collisions may amount to a simple numbers game, some say

Ornithologists in New York first began to take an interest in the phenomenon in 1887 when the newly constructed Statue of Liberty began claiming avian lives.

Today, experts say that migrating birds may die in hundreds or even thousands nightly in cities dominated by skyscrapers.

Yet while Lights Out has been run in Chicago and Toronto with some success, scientists are still unsure exactly why birds hit tall buildings.

Dr Robert DeCandido has been studying bird migration from the top of the Empire State Building for three seasons now.

During this time only four birds were found dead on the observation deck after a rainy night, and no direct collisions were observed.

On some nights birds were confused, but somehow managed to fly by.

He says: "I've been up here on foggy nights, when they circle. And they are very aware of the building. They are avoiding it."

Biggest collision

On rainy nights birds tend to stick together, and often land on the railing of the observation deck of the Empire State after the throngs of tourists leave.

If there are a lot of birds migrating at night, there might be a small percentage doing dumb things

Dr Robert DeCandido

Then they will fly on without a problem.

Dr DeCandido thinks the explanation for collisions with the buildings may amount to a numbers game.

"If there are a lot of birds migrating at night, there might be a small percentage doing dumb things," he says. "You want to prevent it, but sometimes it's not preventable."

The single biggest recorded bird collision in New York happened in 1948, when about 800 birds died after flying into the brightly illuminated Empire State Building on a foggy night.

But on one night in the 1970s, hundreds of birds bumped into the Empire State when the illumination was off, putting into question if the lights are an issue.

Still, collisions with glass remain the principal source of the avian death toll.

Saving energy

In the US, cats kill hundreds of millions of birds a year. Hunters take another 120.5 million, estimates suggest.

But in nature from 60% to 80% of birds do not live to see their second season anyway, killed by predators or dying in migration, especially during hurricane-heavy periods.

Whether a significant step in preventing urban bird collisions or not, Lights Out is helping the birds anyway. Burning less energy helps to preserve their habitat.

Amid soaring energy prices it also makes business sense.

A building with 2.5 million square feet of floor space, or about a third of the Empire State Building, would save up to $120,000 by dimming all of its lights after midnight in the autumn, according to estimates by the Audubon Society of New York City.

The frugal Empire State Building has been doing it since 1975, and now it may be time for others to join.